1. Field of the Invention
The present invention relates to a polyethylene terephthalate monofilament fiber which gives a stress-strain curve exhibiting an elongation of at least 2.0% or more in a stress range of from 10.0 g/d to the point of break. The monofilament fiber according to the invention is characterized by high tenacity, high modulus and low strain, and thus can be used for the production of high tenacity polyester fiber for industrial use, which is used as the material for industrial rope, reinforcement material for construction, webbing or seatbelt.
2. Description of the Related Art
As a useful conventional method for enhancing the tenacity of polyester fibers for industrial use, there is available a method of melting a high viscosity chip having an intrinsic viscosity of 1.0 or greater, heating the melt polymer to a temperature of 310° C. to sufficiently melt the polymer, solidifying the polymer at a quenching temperature of 15 to 18° C. in a hood of 280 mm long at a hood temperature of 340° C., winding the polymer at low speed on godet rollers to obtain undrawn yarn, drawing the undrawn yarn directly in a first step and a second step up to a draw ratio of 6.0, and then relaxing the drawn yarn to finally wind the drawn yarn. Here, the characteristic of high tenacity is obtained by decreasing the degree of orientation of the undrawn yarn through low speed winding, and by drawing the undrawn yarn at a high draw ratio. The polyester yarn produced by the conventional method as described above has a modulus value of 60 g/d to 100 g/d, a stress of 9.5 g/d or less, and an elongation at break of 13 to 18%.
When the draw ratio is increased to obtain a fiber of higher tenacity using such conventional spinning technology, a processing problem of yarn break during spinning and fluffing frequently occur, resulting in poor post-processing properties. Therefore, the conventional technology leads to an increase in the production costs and lowering of the product quality, and thus it is difficult to obtain high tenacity yarns therefrom.